Operating point control for thermionic devices



.May 4, 1937.

G. W. FYLER OPERATING POINT CONTROL FOR THERMIONIC DEVICES Filed Feb. 9, 1935 Inventor: George W. gler,

by# mwa. "y :l/A-ttotnefg- Patented May 4, 1937 OPERATING'POINT CONTROL Fon THERMIONIC DEVICES Ge'orgeWf. Fyler, Schenectady, N. Y., a'ssi'gnorttv` General Electric .Goinpam l a" corporation* of ApplicationFebruary. 9, 1935, Serial. No. 5,744.

6` Caims.-v l(C1. ris- 121151 My' invention relates tov amplifier and modulator devices for .radio..'and=l. analogous systems..

particularly to amplifiers.. and modulatorssof. the

on the characteristic ofV amplifiers-and' modulators of. this type vin such mannenas toincrease .the output and overall eiiciency. ofl systems employing such amplifier or modulator. devices.

1,', `In class A'operation. oanv amplifier .or modulato'rthe linear,.or substantially lineanportion of the. characteristicis utilizedthe bias. voltage appliedto the '.thermionic tube..being such. that a definite anode or plate current iiowseven .when

no signalvoltage is impressed on the input circuit of the tube, Whereas in classB operation the tube is so biased 'that' when .no signal is impressed on the input 'circuitg'the platecurrent is'reduced substantially tozero;

c, Class -B4 operation of amplier 'and modulator.` devices has come'into use as a means ofobtaining higher efficiency and Ymore audio signal .from a.

given'tube: It has 'been found, however, that in many'cases diiiiculties have been encountered in the use of the classB methodof operation dueto distortion of theoutputunder certain conditions,

and other'causes; Witli` class A operation ,of amplier rand lmodulator tubes the above-mentioned distortion and other" difculties encountered in classB operation'f-are largely. avoided.; But'the dissipation-rating "of tubes "operating class A is limited in practiceby the'possibility of the formation-fof hot'fspotsin the 'tubeswhen' operated in thislman-nerf 5 However, thisVx latter vdifculty vis not insurmountable=since,v=in class Aroperation, it is notnecessarya-.fwhen ino signal voltage is impressed on theinput circuit of the'tubefthat the ,full

maximum vanodevoltage be applied vto the tube,

or that-fthe plate current-be the same as under maximum-signal voltage conditions; arrangementsmayin fact-be such, in class A operation, that under't-hei'zonditionv lof minimum or zerosignal, the tube operates on a limited char tice, the 'average input fsignal in .the system is` sub- 55 stantiallyilessfthanimaximum, thereforefby the The circuit acteristic, the i operating i f point being changed'V from: .theloperating l point for maximum `v signal voltage-...to alpointfor minimum or zero signal voltage suchthat the .plate-current when the -tube is operating atthis latter'. pointis-relatively small. For :signal ivolta ges vbetween:maximum and mini;`

above-mentioned, changing-Y of .the yoperating..-4 point, the -plate current is. held-nto. a relativelyA efficiency is thereby. effected.I

Theoperatingfpointof the amplifier, or modusmallaveragevalueand an increaseinoverall class A type, and its objectristo provide means 5 for and methods of changing the `operating point lator,..tube.may .be varied in several Ways to ob. ta.in..the Vabovedescribed,.result. For. example, the change .,in. operating point .may .be made at substantially..constantplate .voltageby a change only inthe bias voltage.. Preferablyghowever, in accordance .with myinvention;v .the change in operating v.point of the ,tubeis made .by varying both the biasfvoltage and the .platevoltage This variation of. thebias voltage..and-plate voltage.

is arrangedto occur in accordance witlithe variai tionsin the amplitudeof the envelope of the modulating. orsignal potentials .impressed on the. audio .orv low frequencyinput Acircuit of the .sys-

tem.. The variationsin amplitude.. of the 'audioA signal envelope ,occur in a. range of frequencies,

.including syllabic. 'frequencies` (so called because their frequencyis substantiallythatof the occur,- rence of syllables in speech) Wlriich are below substantiallyithe minimum .range of frequencies .of the modulating potentials corresponding, `to voice and music frequencies, this lminimum being ,substantially of the order of to 30 cycles A.per second. In a, modification. of my invention the change in operating.point.of. the tube `is -made .at

constant bias voltagelby' varying .only the ,plate Voltage in accordancewiththe.variations in the amplitude of theV envelope'of'the impressed audio signal.

My linvention will be better understood .from the following -'description' when considered in connection 'with the .accompanying'dravring and its scopewill be pointed out in the ,appended claims.

Referringto the drawing, Fig-'"1'- illustrates curves showing ways in which'the operating pointw of a'class-A orsirnilarfamplifierflor modulatory may be changed as the ltube isfoperated between `maximum signal condition andminimum vor: zerosignalzfcondition;and .FigJZll isia circuit t diagram .fV

illustrating a:radinitransmittensystemfini which my invention has;been=lembodiedf-" the operating l In Fig. '1, the Ypoint;Aiirepresents;

point -for maximum signal; of; aiclass amplifier,... or modulator tube'xor devicefand-i-thezlpoints. B1,

B2, B3, B4 andBs indicateaoperating pointsfor zero signal.-. :The clfiange-fofthe operating. point from A to B1 is made by a change in the gridbias voltage impressed. on the *,tubeathe.- plate lvoltage beingV .-.maintained substantially,x 1 constant. In

this .method .of changing..the ...operatingpoina the external plate impedance of the tube must be low for direct current, the resistance of the audio frequency choke coil in the plate circuit being necessarily low for this purpose. The change from A to B2 is made by varying the plate voltage, the grid bias voltage being maintained substantially constant, and the change from A to B3 is made by varying both the grid bias and the plate voltage. The changes from A to B4 and to B5 are made by a change in grid bias alone when the external direct current impedance of the tube is approximately the same as thealternating current impedance.

In the radio transmitter apparatus illustrated in Fig. 2 a radio frequency carrier supplied from any suitable source (not shown) is impressed on the input circuit I0 of a radio frequency power amplifier II the output circuit I2 of which is connected to an antenna. Modulating potentials in the usual audio or voice and music range of frequencies, having the usual minimum frequency of the order substantially of 25 to 30 cycles per second and supplied from -a microphone or other suitable source (not shown) are impressed on the inputl circuit I3 of an amplifier I4. Since the intensity of the sound, in the audio or speech and` music range of frequencies, impressed on the microphone is not ordinarily constant but varies at a relatively slow rate, therefore the amplitude of the envelope of the audio signal potentials impressed on the input circuit I3 correspondingly varies, this variation occurring :at relatively low frequencies, including the abovementioned so-called syllabic frequencies, below the minimum of the audio frequencies. The output of amplifier I4 is impressed, as through leads I5 and input transformer I6, on the input circuit I'I of a class A modulator I8, the plate circuit I9 of which is connected to the plate circuit I2 of radio frequency power amplifier II. In order to provide a self-biasing means for the modulator I8, the grid-cathode or input circuit I1 comprises a resistance-capacity network which will pass these relatively low frequencies, this network having a resistor 26 in series inthe circuit and a capacity 2| in parallel with the resistor.

Plate voltage for the power amplifier I I and the modulator I 8 is supplied preferably from a suitable A. C. power source (not shown) through a main rectifier, indicated generally by the numeral 22, comprising the rectifier elements or tubes 23 and 24 having anodes 25 and cathodes 26, the latter tubes 24 further including grids or control electrodes 21. To prevent disturbing pulsations from the rectifier 22 from being impressed on the plate circuits of modulator i3 and power amplifier II, a filter means 28, including a choke vcoil 29 and a condenser 36 is cnnected between the rectifier 22 and the tubes I8 and II, the cutoff frequency of this filter being in the neighborhood of 20 to 25 cycles. As the power source supplying rectifier 22 is preferably of a usual commercial frequency, for example 60 cycles, the filter 28 is adaptedV to remove the 'ripple due to the '60 cycle supply. 'Ihis filter however, having the cutoff frequency above stated, offers negligible impedance to frequencies below this minimum. The rectifier 22 is connectedv to the plate circuit I9 of modulator I8 through the laudio choke 3|. l

In order to vary the plate voltage of modulator tube I8 and amplifier tube II in accordance with the variations in the amplitude of the envelope of the signal voltage supplied to the audio input circuit I3 from the microphone or other signal source, a control means is provided for varying the output of the current source which supplies plate current for the tubes f3 and II, this current source in the present embodiment of the invention being the main rectifier 22. 'Ihe abovementioned control means comprises a recti1ierfilter circuit including a rectifier 32 and a lowpass filter 33 having a cutoff frequency sufiiciently low to offer a high impedance to frequencies in the audio signal or speech and music range but offering negligible impedance to the relatively low frequencies which characterize the variations in amplitude of the audio signal envelope. The input of the rectifier and filter circuit is connected to the audio amplifier I4 through a transformer 34. The control means further includes a resistor 35 connected across the output terminals of the rectifier-filter circuit. One extremity of resistor 35 is connected to the positive end of a constant grid bias voltage means, as battery 36, the negative end of which is connected to the grids 27 of rectifier tubes 24. The other eX- tremity of resistor 35 is connected to the cathodes 26 of tubes 24 through lead 31. The total bias voltage impressed on grids 2l is, therefore, theV voltage determined by the fixed voltage of battery 36 and the varying voltage drop` across` resistor 35, the larger the drop across resistor 35 the less negative the grid bias impressed on grids 27. A voltage limiter means is connected across resistor 35 comprising a rectifier 38 and a fixed voltage means, as abattery 39, in series with the rectifier 38.

In operation of the radio transmitter system shown in Fig. 2, thecarrier frequency impressed on power amplifier II is modulated in the usual manner by audio frequenciesfrom modulator I6 transmitted thereto from the microphone or other signal source through audio amplifier I4. A. C. power, three-phase in the present instance, which is impressed on the rectifier 22, is rectified in the rectifier 22 and filtered in lter 28, and the resulting direct current is supplied to the plate circuits I2 and I9 of the tubes II and I 8.

As a result of impressing the audio signal voltage from transformer 34 on the rectifier 32 a pulsating direct currenty tends to pass the latter rectifier, or, in other terms, a current tends to pass the rectifier 32 having a direct component and alternating components. One set of alternating components corresponds to the audio frequencies in the voice and music range, for eX- ample the range above substantially 25 to 30 cycles. Another set of alternating components corresponds to the recurring variations in amplitude of the envelope of the audio frequencies. Since, however, 33 is a low pass filter offering a high impedance to the frequencies in the usual audio range, above 25 to 30 cycles, but offering negligible impedance to the relatively slow variations in amplitude of the audio signal envelope, therefore the alternating components in the range above 20 to 25 cycles are suppressed and only the components corresponding to the variations in the amplitude of the audio signal envelope are allowed to pass. Consequently a direct current is caused to fiow through the resistor 35, this direct current varying from a maximum to a minimum in accordance with the envelope amplitude variations. During a period of maximum amplitude of the envelope the direct current fiowing in resistor 35 is a maximum and the drop -of the audio signal envelope.

amarrar resistor 35 is so poledv with reference to bias battery 36 that the drop through the resistorA 35 opposes the voltageof the latter` bias battery.

Therefore, at the assumed period of maximum i amplitude' of theenvelope of the 'audio input potentials, the resultant negative bias 'voltagew" PVAimum or zero amplitude of the envelope.

In a typicalapplication of the transmitter system'A illustrated in Fig. 2, andoperating'on the line connectingA, B3 ofFig. i1, let it be assumed on grids 21 is decreased to its minimum Value.

rectiiier 38 in seriesacross resistor"3`5, operates to prevent greater voltagedrop across"'the"re sistor than is determined,v by the vvoltage'of bat#l tery 39, this battery being so poled with reference to the 'resistor 35 as to tend to'o'ppose the'voltage drop inthe resistor.

The" negative` bias voltage'fdue to battery 36 and 'the drop'through resistor'35, impressed on gridsk21 of main--rectiiier tubes 24,being at its minimum, the voltage impressed by the main rectiiier`22 on the plate circuits of the modulator and amplier tubes I8 and I I is at its maximum;

the decrease in the negative bias on grids 21 having the eiect of increasing the output of the rectiiier 22 which supplies the plate circuits of tubes II andV I8. The lter 28 permits the increaseof output'since 28 is a low-pass filter offering negligible impedance to frequencies corresponding to the relatively slow variations in the amplitude Since, in modulator tube' I8, the plate current which ows through vresistor. in s'eries in the plate-cathode circuit is Vat 4its maximum, vthe voltage drop through this 'resistor 20 is high and the negative `grid bias voltage in tube` I8, due to the drop in resistor 28, is at its maximum.

At the above-assumed period of maximum amplitude of the `audio signal envelope, with the am- ;plier and modulator plate voltage at its maxi- `mum and the negative 'grid bias on modulator tube I8 at its maximum as above described, the operating Vpoint for class A modulator tube I8 is designated in Fig. l by the point A represent- `ing the operating point for maximum" signal.V

If now the amplitude of the envelope of the audio signall potentials impressed in inputcir- :cuit 'I3 is decreased to its minimum, or to its zero value, the direct current in the resistor 35 cor- :respondingly decreases, the value of the nega-v tive bias voltage on the grids 21 of tubes 24 thereby increasing to the value determined by grid bias battery 36 alone. Therefore'at the period of minimum vamplitude of the envelope of the audio signal'potentials, the negative bias voltage yon gridsY 21 is increased to its maximum value. Since the negative bias on grids 21 is now at its maximum, the plate voltage supplied by main rectier 22 to the plate circuitsof tubes I`I and I8 is at its minimum, the increase in grid bias in the rectiiier 22 having the eiect of decreasing the output of this plate supply source. The plate current in modulator tuber I8 now being at its minimum, the voltage drop through resistor ,20 is low and the negative grid bias voltage in tube I8 is at its minimum.

At the assumed period of minimum amplitude of the envelope of the' audio signal potentials, the plate voltage'of amplierI I and modulator I8 being at a minimum and the negative grid bias of modulator I8 also at a minimum, the operating point of class A modulator I8 is shown in Fig. 1 bythe point B3 which represents the point for minimum `or zero amplitude of the envelope of the audio signal potentials;

From the foregoing description of the operation of the radio apparatus 'illustrated in Fig. 2,

it willbe understood 'thatvrith"variationv in am- I plitude of the envelope of the audio signalpo.

tentials, the operating point of class'A modulator tube I8 changesfautom'atically along the'line 40 connecting-the'point 4A of Fig. l, representing i maximum amplitude .of the envelope of the audio signalv potentials; -andpoint B3, representing minthat the modulated amplifier plate voltage, the modulatorplate voltage and plate current are reduced 50% at zero modulation. Also letit be assumed that the plate voltage rises to normal "in proportion to the 'amplitude of the audio modulation. The following table gives the plate power for a 500 kw. transmitter under these conditions, the -modulator eiciency being'assumed to be 40% with '100%modula`tioni Y Power am- Total Percent Modulator Thus if the average modulation during operation of the'system is 25% ,"the ntotal'plateinput power is 634 kw. for the modulator andvmodulated power amplifier. The' reduction ofthe radio frequency carrier to 50% of its maximum or1normal value will not cause noticeable distortion in a receiver which picks `up the signal radiated from the transmitter. With automatic volume control the apparatus illustrated herein provides a substantially distortionless. systemfor Vreducing peaks of modulation, thereby preventing to a certain extent overloading distortion in receivers. It should be noted that when the .transmitter sys` tem is operating at the minimum signal operating point, the modulator and modulatorpower amplifier is capable of'twice the average modulation (assumed as 25%) and` one-half the maximum modulation amplitudes without undue distortion.`v` 'I'his tends to prevent eiectively overmodulationof the transmitter on sudden-'modulation peaks.

A transmitter powered-similarly to the aboveassumed system but having class B modulator apparatus takes the following power,A assuming the same power amplifier eiiiciency as in therst case, andr 54% modulator eciency at 100% vmodula- Thus from a comparison of the above two'tabulatons,v it will` be seen thatthe transmitter having the class B modulator power amplifier system takes, under the 25% average Vmodulation condi-W. tions, 45% more power than thetransmitter hav ingthelclass A modulator power Iamplifier system.. in accordance with my `'inventionu The 'apparent eicienciesfor the similar class B. and theclass A Cyr amplier and modulator systems are 54.5% and 79% respectively.

With power at a cost of 1 per lew. hour, and considering 18 hour daily programs, the yearly power saving is $18,600 for the herein described system inA accordance with my invention, over the -transmitter having the class B modulator system.

It will be understood that the transmitter apparatus illustrated 4in Fig. 2 may be operated as hereinbefore described to vary the' plate voltage of the power amplifier and modulator tubes in accordance with the amplitude of the envelope of the audio signalpot-entials in the audio input, but without variation of the-.grid bias voltage of the modulator IS. In this case, with variation in amplitude ofthe amplitude of the envelope of the impressed audio signal, the operating point of class A modulator tube I8 and power amplifier tube Il changes automatically along the line 4I connecting the maximum operating point A of Fig. 1 and the point B2, the latter point representing minimum or zero amplitude of the envelope of the audio signal potentials.

In a transmitter operating as above set forth on the ABz characteristic of Fig. 1, with no change in modulator grid bias, the following power values are calculated, assuming a 20% reduction in plate voltage and 20% of maximum. plate current for 0% modulation:

Percent Power am- Total modulaplier kw. lgvgdar kw.

tion input p input From a comparison of the latter table with the table hereinbefore given showing power requirements for the transmitter having class B modulator apparatus, it' will be noted that the latter system draws 10% more power under averagemodulationv conditions than the system in accordance with my invention operating on the ABZ characteristic of Fig. 1.

It will be seen fromthe foregoing description Aof radio transmitter apparatus and methods of operation thereof in accordance with my invention, that as compared particularly with apparatus including class B modulator systems, less audio harmonic distortion and at the same time VHbetter overall-efficiency results from the use of class A modulation devices with varying operating points.

My invention has been described herein in particular embodiments for purposes of illustration. It is to be understood, however, that Vthe invention is susceptible of various changes and modifications, and that by the appended claims I intend tocover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent in the United States is:

1. The combination with a signalling system comprising a thermionic device, means to impress on said system modulating potentials in 'the audio frequency range, said potentials having an envelope varying inl amplitude, and plate voltage supply means for lsaid thermioriic device including a rectifier having a control electrode, of a second rectifier and means to supply currrent thereto from said first-named means. a

lter connected to said second rectifier arrangedto pass only the direct current component and current representing the envelope amplitude van'ations of said current supplied from said firstnamedv means, means to impress bias voltages on said control electrodev'arying in accordance with the current variations passed by said filter thereby to vary the plate Voltage ofsaid thermionic device in accordance with said variations in amplitude of saidenvelope, and means responsive to the plate current of said thermio-nic device' to vary the grid bias thereof.

2. The combination with a modulated carrier wave system comprising a carrier frequency amplifier, a modulator connected to said amplifier,A

means to impress on said system' modulating potentials of audio frequency, said potentials. having an envelope varying in amplitude, and voltage supply means for said amplifier and modu-- lator including a rectifier having a control electrode, of a second rectifier and means to impress voltages thereon from said first named means, a lter connected to said second rectifier arranged to pass those frequency variations only which correspond to said envelope amplitude variations, means to impress bias voltages on said control electrode varying in accordance with the frequency variations vpassed by said filter, and means responsive to theplate current of said modulator to vary the grid bias thereof.

3. The combination with a modulated carrier wave system comprising an audio input circuit, means to impress on said system modulating potentials of audio frequency, said potentials having an envelope varying'in amplitude, a modulator, and means including a rectifier having a control electrode to supply plate'voltage to said modulator, of means to shift the operating point of said modulator from a point corresponding to the maximum value of said modulator potentials to a point corresponding to substantially zero value thereof, said last-named means including a second rectifier and means lto impress voltages thereon from said first-named means, a low-pass filter connected to said second rectifier having its cut-off point below the Voice frequencies of said modulating potentials, means to impress bias voltages on said control electrode varying in accordance with the frequency variations passed by said lter, and means responsive to the plate voltage of said modulator to vary the grid bias thereof.

4. The combination with a modulated carrier wave system comprising a carrier frequency amplifier and means to impress a carrier frequency thereon, an audio input circuit and means toimpress modulating potentials of audio frequency thereon, said potentials having an envelope varying in amplitude, a modulator, voltage supply means for said amplifier and modulator including a rectifier having a control electrode, means to impress on said modulator said modulating potentials from said input circuit, and means comprising said modulator to modulate said carrier frequency in said amplifier with said modulating' potentials, of means to shift the operating point of said modulator from a point corresponding to maximum audio signal impressed on said input circuit to a point corresponding to substantially zero audio signal, said last-named means including a second rectifier and means to impress voltages thereon `from said audio input circuit, a low-pass filter connected to said second rectifier having its cut-olf point below the voice frequencies of said modulating potentials,

means to impress bias voltages on said control electrode varying in accordance with the frequency variations passed by said lter, and means responsive to the plate voltage of said modulator to vary the grid bias thereof.

5. In a signalling system comprising a thermionic tube, a source of modulating potentials of audio frequency, said potentials having an envelope varying in amplitude, a source of plate voltage for said thermionic tube comprising a rectier device including a control electrode, a rectier-lter circuit and means to supply voltage thereto from said modulating potential source, said circuit being arranged to pass those frequency variations only which correspond to said amplitude variations of said envelope, and means to impress bias voltages on said control electrode varying in accordance with the frequency variations passed by said rectier-iilter circuit, thereby to control the plate voltage impressed on said thermionic tube in accordance with said variations in amplitude of said envelope.

6. In combination in a radio system, a thermionic tube having a plate circuit, an alternating current source, a source of modulating potentials of audio frequency, said potentials having an envelope varying in amplitude, a rectifier connected to said alternating current source and comprising a plurality of tubes including control grids, a lter connected to said rectifier adapted to pass only a direct current having superposed thereon alternating components of low frequencies of the order of the frequency of the variations in arnplitude of said envelope, means to connect said filter to said plate circuit, bias voltage means for said grids comprising a constant voltage source and a resistor in series therewith, a rectifierlter circuit, means to connect the input of said rectifier-filter circuit to said modulating potential source, and means to connect the output of said rectifier-lter circuit to said resistor, said rectifier-lter circuit being adapted to supply to said resistor a direct current having an alternating component superposed thereon corresponding to said envelope amplitude variations, Whereby the voltage impressed on said plate circuit varies in accordance with said envelope amplitude variations.

GEORGE W. FYLER. 

